Jérémy Pinat released Ton: Drum Machine, a free drum app with Ableton Link and Audiobus! This just came out a few minutes ago so there's no demos yet,
if one arrives over the weekend I'll plug it into this post.

Ton: Drum Machine iTunes Description:

Ton brings a circular step sequencer and an advanced audio engine to your iPad, enabling you to create rich and modulated rhythmic sequences. Its sound-sculpting capabilities are simple yet incredibly deep and will make even the most basic samples sound unique to your style.